Seamless tubes, in particular those made of steel, are produced in three main shaping steps, e.g. piercing, stretching, and finish rolling. Solid round ingots are typically first pierced in a Mannesmann cross rolling mill and later introduced into the so-called sizing or stretcher-reducing mill for finish rolling. Various processes are known for the intermediate stretching, although the instant invention is specifically directed at the Assel process. The Assel process is generally used for the manufacture of tubes of medium and large wall thickness, particularly ones which require excellent surface characteristics and narrow tolerances, such as, for instance, in the manufacture of roller bearing steel pipes. An Assel rolling mill operates in accordance with the principle of cross rolling over mandrel rods, in which three conical work rolls--mounted obliquely at 120 degree angles apart from each other with respect to the rolling axis--are in engagement with the ingot being rolled. Furthermore, the work rolls are displaceable perpendicular to the rolling axis so that a plurality of tube diameters can be produced on an Assel rolling mill. The work rolls of the Assel rolling mill consist essentially of (i) an entry cone, (ii) a working part (the work shoulder), (iii) a smoothing part, and (iv) an exit and rounding part. The smoothing part has the task of partially loosening the tube from the mandrel rod as well as smoothing the surface of the tube, and leveling out differences in wall thickness.
In the rolling of thin-wall tubes in particular, a disadvantage exists in that as a result of the Assel rolling process a trumpet is formed at the rear end of the hollow billet, the trumpet being generally triangular in cross section. The art recognized term for this is triangulation. Triangulation results in a scrap end on the rolled tube which must be cut off. In the case of particularly strong triangulation, the hollow billet can actually remain stuck in the Assel rolling mill, necessitating opening the rolls, resulting in time delays, expenses and other disadvantages.
The cause of triangulation resides in the tendency towards widening of the billet in the rolling mill and is caused by the nature of the Assel process. If the longitudinal tensile stress in the hollow billet decreases towards the rear end of the material being rolled--since sufficient material to be reshaped is no longer present in the entry cone of the Assel rolling mill--the tendency towards an increase in the proportion of the tangential shaping is increased. This means that the radial shaping has a greater impact on tangential shaping and less on longitudinal shaping. In turn this results in an increase in the diameter. If the tangential speed of emergence between the rolls and the mandrel is greater than the entrance speed of the following roll, then there is an accumulation of material in the free spaces between the three rolls which can lead to a blocking of the longitudinal advancement through the mill.
A process of the aforementioned type is known from PCT-WO 90/00449 A1. It has been proposed therein to solve the problems described above by installing on the entry side of the Assel rolling mill a prereduction device which consists of three or four adjustable prereduction rolls, the axes of which have a slight inclination to the axis of the material being rolled or to the axis of the stand of the Assel rolling mill. This angle of inclination can be variable or fixed, and is based on the average angle of advance of the Assel rolling mill. With this known prereduction device, triangulation at the rear end of the hollow billet in the Assel process can be avoided or reduced. The result is that the hollow billet can emerge, disturbance free, from the Assel rolling mill without it being necessary to open the rolls. A reduction in the diameter and/or wall thickness is effected in the prereduction device at the end of the hollow ingot entering the Assel rolling mill. It is therefore the purpose of this prereduction to reduce as much as possible the causative driving force for the widening of the end, namely the radial shaping in the shaping zone of the Assel rolling mill, at least to reduce it to such an extent that there are no longer any disturbances in the Assel rolling mill or high end losses. It would be greatly desirable to optimize the speed and reshaping conditions and thus improve the effect of prereduction in a process of the aforementioned type.